The present invention is directed to ink compositions and, more specifically, the present invention relates to conductive inks which have the capability or quality of transmitting electrical signals generated by electric field assisted acoustic ink jet printing processes and apparatuses, resulting in, for example, controlled ink jettability which in turn can improve the edge raggedness of the images. The conductivity of a material can be measured in terms of the reciprocal of resistivity, which is the capacity for electrical resistance. The conductivity values of the invention inks expressed as log(pico.mho/cm) and recited herein were measured under melt conditions at about 150.degree. C. by placing an aluminum electrode in the molten ink and reading the resistivity output on a GenRad 1689 precision RLC Digibridge at a frequency of 1 K.Hz. Conductivity expressed in terms of [log(pico.mho/cm)] is calculated from the reciprocal of resistivity. Generally, the invention inks possess conductivity values in the range of from about 2 to about 9 log(picomho/cm), and preferably from about 6 to about 8.5 log(picomho/cm), with a melting point of about 60.degree. C. to about 150.degree. C., and preferably about 70.degree. C. to about 90.degree. C., and which inks are especially useful for electric field assisted acoustic ink jet printing with enhanced jettability, processes and apparatuses, reference, for example, U.S. Pat. No. 5,121,141, U.S. Pat. No. 5,111,220, and U.S. Pat. No. 5,371,531, the disclosures of which are totally incorporated herein by reference, including especially acoustic ink processes, such as an acoustic ink printer for printing images on a record medium.
In acoustic ink printing, the printhead produces approximately 2.2 picoliter droplets by an acoustic energy process. The ink under these conditions should display a melt viscosity of about 5 to 10 centipoise or less at the jetting temperature. Furthermore, once the ink is jetted onto the paper, the ink image should be of excellent crease property, and should be nonsmearing, waterfast, of excellent transparency and excellent fix. In selecting an ink for such applications, it is desirable that the vehicle display a low melt viscosity, such as from about 1 centipoise to about 25 centipoise in the acoustic head, while also displaying solid like properties after being jetted onto paper. Since the acoustic head can tolerate a temperature up to about 180.degree. C., and preferably up to a temperature of from about 140.degree. C. to about 160.degree. C., the vehicle for the ink should preferably display liquid like properties such as a viscosity of 1 to about 10 centipoise at a temperature of from about 75.degree. C. to about 165.degree. C., and solidify or harden after jetting onto paper such that the ink displays a hardness value of from about 0.1 to about 0.5 millimeter utilizing a penetrometer according to the ASTM penetration method D1321.
The inks of the present invention in embodiments thereof can be considered a phase change ink, that is, for example, an ink that changes, for example, the crystal structure of the ink components; a physico-chemical process that is, for example, more physical rather than chemical, from a liquid state to solid state in a suitable period of time, for example, from about 1 to about 100 milliseconds, and preferably in less than about 10, such as from about 2 to about 7 milliseconds; and which inks contain (1) a conductive mixture of an inorganic or an organic salt and an oxyalkylene compound having a melting point of lower than about 120.degree. C., and preferably between about 75.degree. C. to about 100.degree. C., and with a low acoustic loss value of below about 100 dB/mm; (2) an ink vehicle that, for example, can fill the pores of the paper and which vehicles possess a melting point of between about 80.degree. C. to about 120.degree. C., and preferably between about 80.degree. C. to about 100.degree. C., with low acoustic loss, which enables a reduction, or minimization of energy consumption, and which acoustic loss is below, or about equal to 60 dB/mm; (3) a viscosity modifying compound that adjusts the viscosity of ink between, for example, about 5 to about 6 centipoise; (4) a lightfastness UV absorber; (5) a lightfastness antioxidant; (6) and a colorant such as a dye, a pigment or mixtures thereof.
More specifically, the present invention is directed to phase-change acoustic ink compositions comprised of (1) a conductive mixture of an organic salt such as urea sulfate or an inorganic salt such as potassium iodide and an oxyalkylene compound such as oxyalkylene bisamides like N,N'-propyleneoxy-propyleneoxy-propylene)-bis-stearamide; N,N'-propyleneoxy-propyleneoxy-propylene oxy-propylene-)-bis-stearamide; poly(alkyleneoxide) alkylates such as N,N'-(ethyleneoxy-ethyleneoxy-ethyleneoxy-ethyleneoxy-ethylene)-stearate; polyoxa-alkanedioate diesters such as distearyl-3,6,9-trioxaundecanedioate, with a preferred melting point of between about 75 and about 100.degree. C., and a low acoustic loss and which acoustic loss is below about 100 dB/mm, and is preferably in the range of between 25 to about 80 dB/mm; (2) an ink vehicle such as a bisamide including N,N'-stearylene bis-stearamide; an oxazoline compound such as 2-stearyl-5-(hydroxymethyl)-5' (methoxy stearate) oxazoline; carbamate compounds such as tert-butyl carbamate (Aldrich #16,739-8) that, for example, can fill the pores of the paper and which vehicle possesses a melting point of between about 80.degree. C. to about 120.degree. C., and preferably between about 80.degree. C. to about 100.degree. C., with a low acoustic loss, which enables a reduction, or minimization of energy consumption, and which acoustic loss is below, or about equal to 60 dB/mm; (3) a viscosity modifying amide compound such as N,N'-hexamethylene bisacetamide, (Aldrich #22,423-5); N,N'-octamethylene-bis(dichloroacetamide), (Aldrich #14,750-8), that adjusts the viscosity of the ink to, for example, from about 5 to about 10 centipoise, and an acoustic loss in the range of between about 5 to about 40 dB/mm; (4) a UV absorber such as 2,2,6,6-tetramethyl-4-piperidinyl/.beta.,.beta.,.beta.',.beta.'-tetramethy l-3,9-(2,4,8,10-tetraoxo spiro(5,5)-undecane) diethyl]-1,2,3,4-butane tetracarboxylate; (5) an antioxidant such as antimony dialkyl phosphorodithioate; and (6) a colorant, and wherein there can be generated with such inks excellent developed images on plain and coated papers with acceptable image permanence, excellent projection efficiency on transparencies without a post fusing step, and excellent crease resistance, and wherein the inks possess acceptable, and in embodiments superior lightfastness, for example between about 90 to about 100 percent, and superior waterfastness between about 95 to about 100 percent values. Moreover, in embodiments of the present invention there is enabled the elimination, or minimization of undesirable paper curl since water need not be present, or minimum amounts less than 1 percent of water may be selected in embodiments, and it is preferred in embodiments that there be an absence of water. When water is not present in the inks, a dryer can be avoided thereby minimizing the cost of the acoustic ink jet apparatus and process.